The extensively reworked new implementation for specifiying jar pathnames has now been merged to trunk as [svn r12422][1].
[1]: http://trac.common-lisp.net/armedbear/changeset/12422
Excerpts from the commit message:
Pathname namestrings that have the form "jar:URL!/ENTRY" now construct references to the ENTRY within a jar file that is located by URL. The most common use is the "file:" form of URL (e.g. 'jar:file:/home/me/foo.jar!/foo.lisp') although any valid syntax accepted by the java.net.URL constructor should work (such as 'jar:http://abcl-dynamic-install.googlecode.com/files/baz.jar!/a/b/eek.lisp').
The internal structure of a jar pathname has changed. Previously a pathname with a DEVICE that was itself a pathname referenced a jar. This convention was not able to simultaneously represent bothjar entries that were themselves jar files as occurs with packed FASLs within JARs and devices which refer to drive letters under Windows. Now, a pathname which refers to a jar has a DEVICE which is a proper list of at most two entries. The first entry always references the "outer jar", and the second entry (if it exists) references the "inner jar". Casual users are encouraged not to manipulate the "internal structure" of jar pathname by setting its DEVICE directly, but instead rely on namestring <--> pathname conversions.
Jar pathnames are only currently valid for use with LOAD, TRUENAME, PROBE-FILE and pathname translation related functions (such as MERGE-PATHNAMES, TRANSLATE-PATHNAME, etc.) Passing one to OPEN currently signals an error. Jar pathnames do not currently work with DIRECTORY or PROBE-DIRECTORY.
Jar pathnames work for ASDF systems packaged within JARs. We override ASDF:LOAD-OP to load ASDF from JAR Pathnames by bypassing compilation if the output location would be in a JAR file. Interaction with ASDF-BINARY-LOCATIONS is currently untested.
Pathname now used as the basis of ABCL's internal routines for loading FASLs replacing the use of strings, which simplifies a lot of the behavior in looking for things to LOAD.